1. Field of the Invention
This invention relates to the fabrication of hard disk drives (HDD), particularly to a method of protecting a magnetic head and magnetic disks by use of a diamond-like carbon coating on an underlayer that also serves as a corrosion barrier.
2. Description of the Related Art
Reducing the head-to-disk spacing (fly height) between a magnetic read/write head and the surface of a magnetic disk rotating beneath it has been one of the major approaches in achieving ultra-high recording density in a hard disk drive (HDD) storage system. For a commercially available HDD with 160 GBytes capacity, the fly height is on the order of 10 nanometers (nm). Maintaining such a small spacing between a rapidly spinning disk and a read/write head literally flying above it is difficult and an occasional contact between the disk surface and the head is unavoidable. Such contact, when it does occur, can lead to damage to the head and the disk and to the loss of recorded information on the disk. To minimize the head and disk damage, a thin layer of DLC (diamond-like carbon) coating is applied to both the surface of the head and the surface of the disk. This DLC also serves to protect the magnetic materials in the head from corrosion by various elements within the environment. Given the importance of the role of the DLC, it is essential that it is hard, dense and very thin, the thinness being required to satisfy the overall fly height requirement while not using up any of the allotted spacing. Currently a DLC coating between 20-30 angstroms is found in the prior art.
Conventionally, DLC coating thicknesses are greater than 50 Å and for that thickness range, there is a high degree of internal stress, leading to poor adhesion with the substrate materials of the head as well as to other substrates to which they may be bonded. Because of high internal stress and thermal stress, an underlayer is required. For example, in applications of cutting edges and drilling tools, the DLC thickness is in the micron range, and the working temperature can go up to a few hundreds degrees Celsius. Thus, the coefficient of thermo expansion (CTE) of the underlayer also plays an important role. For these reasons, in prior arts, Japanese Patents JP 2571957, JP2215522 and JP3195301 have proposed Si, SiOx, SiC and SiNx for this adhesion layer. Itoh et al. (U.S. Pat. No. 5,227,196) discloses a SiNx underlayer on an oxide substrate beneath the DLC layer. Various other types of adhesion layers are also found in the prior arts. Ishiyama (US Patent Application 2006/0063040) discloses a carbon-based protection layer of hydrogenated carbon nitride for better adhesion. Hwang et al. (US Patent Application 2005/0045468) teaches a Si underlayer for a DLC. Hwang et al. (US Patent Application 2002/0134672) discloses Si, Al2O3, SiO2, or SiNx as an underlayer beneath a DLC layer. David et al. (U.S. Pat. No. 5,609,948) describes a SiC underlayer under a DLC layer.
In addition to this cited prior art, adhesion layers comprising materials other than Si have also been utilized in other areas. Natsume et al. (U.S. Pat. No. 7,091,541) discloses the oxynitride TiAlON for an underlayer between a capacitor dielectric layer and an electrode. Fu et al. (U.S. Pat. No. 6,238,803) shows a TiOxNy barrier layer for an Al electrode. Johnson et al. (U.S. Pat. No. 4,952,904) describes a metal oxide underlayer between silicon nitride and platinum. Stevens (U.S. Pat. No. 5,070,036) shows metal oxynitride as one of various material regions in a VLSI circuit. Gillery (U.S. Pat. No. 4,861,669) shows a TiOxNy dielectric film.
For magnetic heads, the underlayer should have at least the following properties:
1. Electrical isolation property. For magnetic heads, electrical isolation must be provided for the magnetic metal alloy layers, such as those layers comprising a magnetoresistive read head based on the giant magnetoresistance (GMR) effect, or those layers comprising a device based on the tunneling magnetoresistive (TMR) effect. Electrical short circuits between these layers and surrounding HDD components will damage the head or similar device. For this reason, the protection layers, especially the underlayer, should be insulating or semi-insulating. However, due to the semiconductor properties of Si, the surface shunting of a Si underlayer can introduce noise, such as the so-called popcorn noise, into the GMR or TMR reader.
2. Anti-corrosion property. DLC films, particularly those produced through the filtered cathodic vacuum arc (FCVA) process of the prior art, are often embedded with micro- or nano-articles. These particles can result in pinholes and corrosion of the materials used in forming the magnetically active layers, such as NiFe and NiCoFe. The anti-corrosion property of the underlayer is therefore of crucial importance to maintaining the performance integrity of the sensor.
3. Anti-wear property. With the total thickness of the underlayer and the DLC layer being reduced to the sub-30 angstrom range, literally every atom counts for the protection. Thus, a better anti-wear property is expected if we can put more atoms in the limited film thickness. It is therefore very important that the underlayer have both chemical stability for corrosion protection and high hardness for tribological advantage.
It is the purpose of the present invention to provide a new class of materials as underlayers to replace the Si and related materials described in the prior art cited above and to provide the above properties.